だいぶ慣れて来ました
extension BilinearMap: VectorSpace, BilinearMapType where Domain: ProductSetType & VectorSpace, Domain.Left: VectorSpace, Domain.Right: VectorSpace, Codomain: VectorSpace, Domain.CoeffRing == Codomain.CoeffRing, Domain.CoeffRing == Domain.Left.CoeffRing, Domain.CoeffRing == Domain.Right.CoeffRing {